The present invention relates to an apparatus useful to store heat. The heat storage medium utilized in the present apparatus is fusible, that is, it undergoes a solid-liquid phase change during its charging and discharging cycles. During such cycles, the density of the medium decreases (i.e., the volume increases) as the medium melts. To accommodate this volume change, a clearance space is provided which normally contains gas--for example, air. The present invention provides an improved means for alleviating high pressures which may be produced within the heat storage container when the heat storage medium is converted from the solid state to the liquid state. The present invention also provides an improved arrangement of heating units whereby the heating elements may be replaced without draining the heat storage medium from the heat storage container.
When uniform heat is applied to a solid material held in a container, the uniform heat throughout the solid results in the even liquefaction of the solid. The formed liquid can expand into the clearance space, and no adverse internal pressures are built up within the container. However, it is impractical to apply a uniform heat to all parts of a tank of solid heat storage material. In actual practice, localized heaters are utilized, and the problem of internal pressures within the container is encountered and must be reckoned with.
In operation, when the localized heaters are utilized to melt a solid heat storage medium, which may occur either on initial start-up or during any cycle in which the heat storage medium has been allowed to solidify, a large amount of heat is supplied to the heat storage medium at relatively localized points. Such localized heating melts the solid material in pools contained within the otherwise solid material. As the solid melts, the expansion of the liquid phase produces high internal pressures which are capable of distorting or rupturing the container.
Presently, heat storage units are fabricated by positioning one or more main heating units within the heat storage container. These units are usually located in the bottom portion of the container. A vertical heater may be utilized to provide a pressure escape path to the clearance space for the expanding liquid medium. The use of only vertical heaters is not feasible because the heated liquid medium circulates very readily and rapidly rises to the top. Thus, the maximum temperature allowable in the system would be reached at the top while the temperature at the bottom would be considerably lower, and the full heat storage capacity of the medium could not be utilized. The prior art proposes an escape channel by providing a vertical heater which may be a length of metal thermally connected to and heated by one or more of the lower main heaters, or by a vertical section of a main heater which extends to the clearance space. The length of metal or the vertical section of heater extend through the level of the heat storage medium when liquid. The vertical heater section is the more reliable means of providing an escape path, as the heat conductivity of the length of metal may not be sufficient to provide an escape path before high pressure is built up from the lower heater source. However, the vertical heater section, being activated when the unit is discharged, has a substantial amount of surface area exposed to the gas in the clearance space until the level of the melting heat storage medium rises to its highest level. Since heat dissipation from the heater surface to the liquid medium is much more effective than to a gaseous ambient, this can result in serious overheating of the heating element exposed to the gas and in premature failure of the element and rapid corrosion of the heater surfaces exposed to the gas.
A preferred type of heater consists of an electrical heating element, such as Nichrome wire, inside of a metallic tube which is immersed in the heat storage medium. The tube protects the element from corrosion by the medium, and from electrical short-circuiting by the medium, many of which are electrically conducting when in the molten state.
In actual practice, the heaters are arranged within the container, molten heat storage medium is introduced into the container, allowed to solidify, and the unit transported to an installation site. The embedded heaters are then utilized to melt, or charge, the heat storage medium and impart heat into the system. Main heating units of the prior art have commonly been of the type in which the heating element, an electrical insulating material, and a metallic outer sheath, are constructed as an inseparable unit, and are immersed in the heat storage medium. Such designs do not lend themselves to in situ repair or replacement of heating elements. Typical prior art heater arrangements are found in U.S. Pat. Nos. 3,356,834; 3,439,151; 3,453,416; 3,475,596; 3,492,461; and 3,558,856.
Preferred heat storage mediums are alkali metal hydroxide compositions. Such compositions are preferred because of their relatively high storage capacities, high heats of fusion, broad operative ranges, relative inertness, and their low vapor pressures. Alkali metal hydroxides have melting points ranging from about 272.degree. C. for cesium to about 450.degree. C. for lithium. The incorporation of additives such as corrosion inhibitors and non-reducing agents into the alkali metal hydroxide heat storage compositions facilitates the production of useful mixtures with a variety of melting points.
Sodium hydroxide compositions are commonly available and are aptly suited for use as the heat storage medium of the present invention. Relatively pure sodium hydroxide has a melting point of about 318.degree. C. However, compositions including other salts may consist of liquid-solid mixtures in the range from about 232.degree. C. to about 340.degree. C. During a heat storage cycle, sodium hydroxide may be heated to temperatures as high as 675.degree. C. without harm. Normal operating temperatures of heat storage units containing sodium hydroxide compositions as a heat storage medium range from about 100.degree. C. to about 500.degree. C.
The previously proposed heat storage devices present a problem when the immersed heating elements fail. In order to repair or replace the defective element, the heat storage medium must be melted and drained, the element repaired and replaced, and the heat storage medium returned to the container. Such maintenance and repair operations are very costly. As a practical matter, the units requiring repair are usually taken out of service and returned to the manufacturer, entailing the substantial expense of disconnecting, shipping, and subsequently reconnecting of the unit. There is also the added inconvenience of lack of service if a spare unit is not readily available.
The present invention describes an improved vertical heater arrangement which provides an escape channel for the liquid heat storage material within the solid heat storage medium, thereby avoiding the development of internal pressures within the unit due to non-uniform heating, and avoiding deleterious overheating of the heater elements or the protective sheaths of the heaters.
The present invention also provides a heat storage apparatus which facilitates the repair or replacement of heating elements at the use-site, preferably without interruption of the heat storing operation.